Process for the treatment of anodic oxidized aluminum surfaces

ABSTRACT

An improvement in the treatment of anodic oxidized aluminum surfaces with subsequent sealing with hot water or steam. The improvement comprises either an intermediate treatment or a combined treatment with the sealing of the aluminum surfaces at temperatures of from 150* C to 100* C with a solution of dextrin. The treatment prevents the formation of sealing films without impairing the anodic oxide coating or the quality of the aftersealing.

Uite States Patent Germscheid et a1.

[15] 3,657,77 [451 Apr. 18, 1972 PROCESS FOR THE TREATMENT OF ANODIC OXIDIZED ALUMINUM SURFACES Inventors: Hans Gunther Germscheid, l-losel; Roland Geisler, Dusseldorf-Holthausen, both of Germany Assignee: Henkel 8: Cie GmbH, Dusseldorf- Holthausen, Germany Filed: July 8, 1970 Appl. No.: 53,317

Foreign Application Priority Data Sept. 2, 1969 Germany ..P 19 44 452.1

US. Cl. ..204/35 N, 204/58 Int. Cl. ..C23f 17/00 Field of Search ..204/35 N, 42, 58

[56] References Cited UNITED STATES PATENTS 2,376,082 5/1945 Pullen 3,016,339 1/1962 Riou et a1 ..204/35 N Primary Examiner-John H. Mack Assistant ExaminerW. I, Solomon Attorney-Hammond & Littell [5 7] ABSTRACT 4 Claims, N0 Drawings PROCESS FOR THE TREATMENT OF ANODIC OXIDIZED ALUMINUM SURFACES THE PRIOR ART Anodic oxide coatings are frequently applied to aluminum surfaces for the purpose of corrosion protection. These oxide coatings protect the aluminum surface against the effects of the atmosphere and other corrosive media. Furthermore, anodic oxide coatings are also applied so as to obtain a harder surface and thus improve the wear-resistance of the aluminum. By virtue of the color of the oxide coatings or their readily partial colorability, particularly decorative effects can be achieved.

A number of processes are known for the production of anodic oxide coatings on aluminum. The production of the oxide coatings takes place for example by direct current in solutions of sulfuric acid (Direct current-sulfuric acid process). Frequently, however, solutions of organic acids, such as in particular sulfophthalic acid or sulfanilic acid or mixtures of these acids with sulfuric acid are used. The last mentioned processes are known in particular as hard anodizing processes.

These anodically applied oxide coatings do not fulfill all requirements in respect of corrosion protection, however, since they present a porous structure. For this reason, it is necessary subsequently to seal the oxide coatings. This aftersealing is carried out with hot or boiling water or with steam, and is termed sealing." In this way, the pores are closed and thus the corrosion protection is substantially enhanced.

In the after-sealing of anodically applied oxide coatings, not only are the pores sealed, but a velvety film of greater or lesser thickness, the so-called sealing film, forms over the entire surface. This film consists of amorphous aluminum hydroxide and is not resistant to handling, so that the decorative effect of the coating is impaired. For this reason, it has been necessary until now to remove this film mechanically by hand.

OBJECTS OF THE INVENTION It has now been found, that these disadvantages can be substantially overcome if the process described in the following for the treatment of the surface of aluminum or aluminum alloys by anodic oxidation with a subsequent sealing by steam is used.

An object of the present invention is the development in the process for treating aluminum surfaces which comprises subjecting aluminum and aluminum alloy surfaces to an anodic oxidation and subsequent sealing with hot water or steam, the improvement selected from the group consisting essentially of (A) applying a solution containing from 0.1 gm to 50 gm per liter of dextrin to the anodic oxidized surfaces at temperatures of from C to 100 C prior to said sealing and (B) applying said solution to the anodic oxidized surfaces simultaneously with said sealing with hot water.

These and other objects of the invention will become more apparent as the description thereof proceeds.

DESCRIPTION OF THE INVENTION The present invention, therefore, provides a process for the treatment of aluminum or aluminum alloy surfaces by anodic oxidation with a subsequent sealing by water or steam, in which, prior to the sealing, the said surfaces are treated at a temperature of from 15 C to 100 C with a solution containing dextrin.

The dextrin solution preferably contains from 0.1 to 50 gm/l of dextrin.

For the realization of the process, the commercial dextrins may be used. Preferably, the intermediate treatment is carried out with solutions containing dextrins of a viscosity of 50 to 400 c? in 50 percent solution at C. The viscosity is measured with a Brookfield rotary viscometer.

The solution for the intermediate treatment contains 0.1 to 50 gm/l of the said dextrins, which may be used singly or in mixtures. It is also possible, of course, to use larger quantities, but this offers no further advantages.

The above-mentioned solutions for the intermediate treat ment have a pH value situated between 5 and 7. This pH value has proved appropriate and is the natural pH of said solutions, so that additional pH adjustment is not required. For the preparation of the solutions, it is advantageous if fully demineralized, or distilled or condensed water is used.

The intermediate treatment is carried out at temperatures between 15 C and 100 C, preferably at temperatures above 50' C. The anodized articles of aluminum or aluminum alloys are dipped into the solutions or are sprayed with the same. The duration of treatment is from 0.5 to 60 minutes, generally less than 15 minutes. Longer treatments have no detrimental effect. After the intermediate treatment with the abovedescribed solutions, a rinsing with water may take place to avoid an entrainment of substances removable by rinsing into the subsequent sealing stage. This rinsing is not essential however. No disadvantages result if the aluminum parts are transferred, immediately after the intermediate treatment to the after-sealing stage.

In a particular embodiment of this process, the after-sealin g with water and the intermediate treatment are carried out in one operating stage. In this case, the water provided for the after-sealing is mixed with the said dextrins in the specified quantities and the scaling is carried out in the customary manner. It is appropriate subsequently to rinse with fully demineralized water or distilled or condensed water.

The after-sealing solutions may also contain additives known for such purposes, such as nickel acetate, in small quantities. It is possible by the new process to prevent the formation of sealing films without impairing the anodic oxide coating or diminishing the quality of the after-sealing. The appearance of the surface is not affected by the process in accordance with the invention. The effects, as achieved by the pretreatment and anodizing, remain unchanged.

The following examples are illustrative of the practice of the invention without being limitative in any manner.

In the following specific examples the quality of the oxide coatings is determined by the so-called Testal value according to DIN 50,949. Furthermore, the quality of the sealing was tested by the Griin test according to DIN 50,146. The designation of the aluminum alloys used in the examples is in accordance with DIN 1,725. Testal values above 15 are technically not satisfactory. DIN is the abreviation for Deutsche Industrie-Norm representing a series of standard German published test procedures.

EXAMPLE 1 Aluminum shaped specimens (A1 Mg Si 0.5), alkaline degreased and pickled in the customary manner, which had been anodically oxidized by the direct current-sulfuric acid process (thickness of coating 20 [.L), were treated for 10 minutes with a solution of 5 gm/] of dextrin having a viscosity of cP (measured in 50 percent solution at 20 C with the Brookfield rotary viscometer), in fully demineralized water at C. After subsequent sealing in fully demineralized hot water C, 60 minutes) the sections showed no sealing film. The thickness of layer after sealing was 20 [.L, the Testal value had dropped from over 300 to 6.5. The Grt'in" test showed a perfect sealing.

EXAMPLE 2 Aluminum sheets (Al Si 5), degreased in the customary manner, which had been anodically oxidized by the direct current sulfuric acid-oxalic acid process (thickness of coating 10 10 t), were treated with a solution of 10 gm/l of dextrin having a viscosity of 275 c? (measured in 50 percent solution at 20 C with the Brookfield rotary viscometer) in distilled water at 50 C for 15 minutes. After intermediate rinsing, the aluminum parts were sealed in steam for 60 minutes. After this, the sheets showed no sealing film. The thickness of layer after sealing was 10 p. and the Testal value had dropped from over 300 to 16.0. The Gn'in"test indicated a perfect sealing.

EXAMPLE 3 Aluminum shaped specimens (Al Mg 3), 3), alkaline degreased and pickled in the customary manner, which had been anodically oxidized by the hard anodizing process (thickness of coating 29 [.L), were sealed for 60 minutes in a solution of l gm/l of dextrin having a viscosity of 200 cP (measured in 50 percent solution at 20 C with the Brookfield rotary viscometer) in fully demineralized water at 100 C. Subsequently, the parts were rinsed briefly with fully demineralized water. The sections showed no sealing film. The thickness of the coating after the sealing was 29 u. The Testal value had dropped from over 300 to 400. The G rt'intest also indicated a perfect sealing ofthe Eloxal coating.

EXAMPLE 4 Aluminum sections (Al Mg Si 0.5), alkaline degreased and pickled in the customary manner, which had been anodically oxidized by the direct current-sulfuric acid process (thickness of coating 20 to 22 ,u), were sealed for 10 minutes at 100 C in solutions containing 5 gm/l of the chemicals mentioned below, or they were treated in these solutions at 80 C for minutes and then, after an intermediate rinse, sealed in hot water at 100 C for 60 minutes. The thickness of the coating remained unchanged on the different sections after the sealing. The formation or prevention ofthe sealing film and the varying quality of the sealing, determined on the basis of the Testal value are summarized in the table. Only when the compounds in accordance with the invention were used, a sealing film did not form and any detrimental effect on the after-sealing was avoided.

TABLE Tcstal value DIN 50949 after sealing Glucose not prevented Saccharose 5.0 not prevented Starch. low viscosity 21.0 partly prevented Starch, high viscosity l3.0 partly prevented Carboxymethyl cellulose l2.0 partly prevented Hydroxyethyl cellulose l 1.5 not prevented Carboxymethylmethylcellulose l0.0 partly prevented Dextrin (60 c? 6.0 prevented Dextrin 150 c? J 5.0 prevented Dextrin (300 cp 5.5 prevented Dextrin (380 c? 6.5 prevented The viscosity was determined in 50% solution at 20 with the Brookfield rotary viscometer.

The preceding specific embodiments are illustrative of the practice of the invention. It is to be understood, however, that other expedients known to those skilled in the art may be employed without departing from the spirit of the invention or the scope of the appended claims.

We claim:

1. In the process for treating aluminum surfaces which comprises subjecting aluminum and aluminum alloy surfaces to an anodic oxidation and subsequent sealing with hot water or steam, the improvement consisting essentially of applying a solution containing from 0.1 gm to 50 gm per liter of dextrin and having a pH between about 5 and 7, to the anodic oxidized surfaces at temperatures of from 15 C to C prior to completion of said sealing. 2. The process of claim 1 wherein said dextrin has a VISCOSI- ty of from 50 to 400 c? measured in a 50 percent solution at 20 C.

3. The process of claim 1 wherein said application of said solution is at temperatures of over 50 C.

4. The process of claim 1 wherein said solution of dextrin is applied to said anodic oxidized surfaces simultaneously with said sealing with hot water. 

2. The process of claim 1 wherein said dextrin has a viscosity of from 50 to 400 cP measured in a 50 percent solution at 20* C.
 3. The process of claim 1 wherein said application of said solution is at temperatures of over 50* C.
 4. The process of claim 1 wherein said solution of dextrin is applied to said anodic oxidized surfaces simultaneously with said sealing with hot water. 